update input value mappings

This PR closes #314 and clarifies the `value` attribute mappings for other input types.
note: the `value` attribute only represents *the initial value* for the input elements that expose the attribute's value.  If a user updates a form control the `value` attribute's value does not mutate to reflect the updated entry.  The updates I provided for these inputs attempts to specifically call this out.  if this is not clear, please suggest alternate wording.

Note that the `input type=color` `value` is far more complex than the other inputs and I would appreciate specific feedback on what would need to be updated here, if anything.

Additionally, the `input type=color` seems to make a strong case for allowing `aria-valuetext` on a `button` element – where the button should be named via a `label` or an `aria-label/labelledby`, but still needs to expose it's displayed value (subtree of the button element).

Author: scottaohara